Morphology: Enveloped, double-stranded DNA virus. Family Herpesviridae, subfamily Alphaherpesvirinae. Primary infection = varicella (chickenpox); reactivation from dorsal root ganglia = herpes zoster (shingles).
Typical drugs
- #1Valacyclovir— **1 g PO TID × 7 days** for herpes zoster — preferred outpatient. Start within 72 h of rash onset for max benefit.
- #2Famciclovir— **500 mg PO TID × 7 days** for zoster — equivalent to VACV.
- #3Acyclovir— PO 800 mg 5×/day adequate but adherence poor. IV 10 mg/kg q8h for disseminated zoster, ophthalmic zoster in immunocompromised, or encephalitis.
- #4Foscarnet— ACV-resistant VZV in immunocompromised.
Empiric therapy when resistant
ACV-resistant VZV → foscarnet. Standard cases respond well to first-line nucleoside analogs.
Resistance mechanisms
altered-target
TK mutations → ACV resistance
Example: Rare; immunocompromised on chronic suppression. Foscarnet for salvage.
Resistance notes
Very rare resistance in immunocompetent. ~5% in immunocompromised on chronic suppression.
Pearls
Herpes zoster (shingles) = dermatomal pain ± vesicular rash, often unilateral. Hutchinson sign (tip of nose) → V1 / ophthalmic zoster — emergent ophthalmology. Disseminated zoster = >20 vesicles outside primary dermatome — IV ACV + airborne+contact precautions until crusted. Post-herpetic neuralgia (PHN) affects 10–20% older patients; gabapentinoids, TCAs, topical lidocaine / capsaicin. Shingrix vaccine (recombinant adjuvanted, 2-dose) for adults ≥50 + immunocompromised ≥19 — much more effective than older Zostavax (now discontinued). Varicella in adults is more severe than children — risk of pneumonia, hepatitis, encephalitis; ACV + airborne+contact precautions. VZV vasculopathy can cause stroke months after zoster — get MRI/MRA if focal neuro symptoms.